User blog comment:Anobi/Alternate Timeline Theory/@comment-5621513-20140721183325/@comment-25110963-20140725065323

It's possible Nero might be a half brother of sorts. I can't really imagine Sparda falling in love with another woman as he did Eva unless we are officially told otherwise. Here's my theory. Somewhere before Nero was born, he was infused with some latent demon power or blood of Sparda. That would explain the hair and right arm. Now full demon power does not awaken until they are mature enough to activate Devil Trigger. The Order probably had access to one of Sparda's weapons, relics, or some sort of personal effect that contained his power. That power somehow transferred to Nero through his mother. Maybe a follower or maiden or whatever. I don't know how it transferred but it did. She probably died when Nero was still young and was left to be found. Then he lives a life with Kyrie and Credo and etc., etc., the story catches up to the current events if DMC4.